1. Field of the Invention
The invention relates generally to a method and apparatus for real-time filtering of position error signals (PES) in disk drive servo systems that minimizes the time delay shift between the raw position error signal and the filtered output.
2. Description of the Prior Art
Computers often include mechanical moving storage devices, e.g., disk or tape drive units, having media on which data can be written and from which data can be read for later use. Disk drive units that incorporate stacked, commonly rotated, rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces on the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
All disk drive units must have a method to position each transducer head over the proper radial location to write a track and again, to position it very close to the same location to read the track. With the higher level performance disk drive units, e.g., those that use a voice coil type of actuator, a feedback mechanism must be provided to locate and stably hold the head on a given track. Typically, track accessing and track following are provided utilizing a magnetically pre-written pattern on the disk surfaces in the disk drive unit. A dedicated servo system employs only one surface of one of the disks in the disk drive on which is pre-written all the magnetic servo patterns used for position error information required by the tracking and track accessing servo. A sector servo system, in contrast, uses small portions (typically, 10-15%) of each of the data surfaces for magnetically pre-written servo patterns. These servo patterns are written on the disk surfaces within even-spaced radial servo sectors that are interlaced with data sectors. The radial servo sectors provide tracking and track access information for all concentric data tracks written on the particular disk surface. A hybrid servo system uses both dedicated servo and sector servo to obtain the advantages of each type of servo system.
Precise tracking and fast accessing of the transducing heads to any track are crucial to high performance disk drives. These requirements call for a closed loop servo system that has high degree of servo stability. A high degree of servo stability means that the radial motions of the transducing heads are smooth and not jerky, so that the transducing heads do not waver back and forth. In a similar situation, an automobile with worn shock absorbers will waver up and down when the wheels hit a bump on the road surface making steeling difficult, while an automobile with very stiff shock absorbers will transmit the shock of the bump to the driver making the drive harsh. An automobile with a high degree of servo stability, on the other hand, strikes a balance between the two extremes. One measure of servo stability is the phase margin (PM). The PM may be defined as a measure of the additional phase lag or time delay that can be tolerated in the servo loop before instability results. Servo instability in a disk drive is a catastrophic event. It will cause the transducing heads to waver radially back and forth with increasing amplitudes on the disk surface until the disk drive eventually self-destructs.
The servo system controlling the tracking and accessing of the transducing heads in disk drives uses a position error signal (PES) obtained by decoding prerecorded magnetic servo patterns on the disk surfaces. Noise in the decoded PES is a common problem in disk drives as disclosed in U.S. Pat. No. 4,412,165, issued to Case et al. One noise reduction method using common averaging provided by an analog linear filter is discussed in U.S. Pat. No. 4,380,034, issued to Krake. U.S. Pat. No. 4,982,297, issued to Tsujisawa, uses a real-time linear recursive digital filtering method for compensating for erroneous detection of PES. Real-time digital recursive filters, such as that used on the Tsujisawa patent, derive the next output from a linear, weighted sum of previously stored filter input values and output values.
Another technique for removing noise from the PES is simply to "ignore" PES samples that seem to be erroneous. Rather than utilizing a real-time linear filter that repetitively averages the last M position error signal (PES) samples, a real-time M-point median filtering method may be used. Here, the value M is typically a small odd integer. A real-time M-point median filter is a nonlinear filter that will pick the middle value from the last M samples. For example, the middle or median value of the last three PES samples could be extracted using a real-time 3-point median filter. A median filter is a nonlinear filter that will pick the middle value from an odd number of samples. The use of a real-time median filter in connection with the position error signal (PES) in disk drives has been disclosed in T. Makansi, "Median Filtering of Position Error Signals in a Data Recording Disk File," IBM Technical Disclosure Bulletin, Vol. 29, No. 12, May 1987, pp. 5239-5240. However, the Makansi disclosure only teaches finding the median of three or more sampled values of past PES measurement values. Any real-time filter used in a closed loop servo system, no matter whether linear (such as those disclosed in the Krake and Tsujisawa patents discussed in the previous paragraph) or nonlinear (such as that disclosed in the Makansi patent), that uses only past PES measurement values as inputs in the filtering operation adversely decreases the phase margin (PM). This occurs because an additional time lag is added to the servo system, and the servo tracking system is therefore subject to reduced stability.
Other methods for improving the tracking capability of servo systems in the presence of PES noise, e.g., caused by disk runout and the like, include the use of feedforward techniques in the disk drive servo. The basis for such feedforward techniques is that a variable portion of the dynamic PES can be anticipated. Thus, near term future behavior of the PES can be predicted with some accuracy. The PES is considered to consist of two components, the repeatable or periodic PES and the non-repeatable or non-periodic PES, being synchronous or non-synchronous with the disk rotation, respectively. The PES samples for a given track on each disk surface are averaged per sector over several revolutions of the disk pack. The resultant averaged PES samples, often referred to as the repeatable PES, are used by the servo system to anticipate synchronous motions of the head. The non-repeatable PES is obtained by subtracting the "raw" PES from the repeatable PES. U.S. Pat. No. 4,412,165 issued to Case et al.; U.S. Pat. No. 4,536,809 issued to Sidman; U.S. Pat. No. 4,594,622 issued to Wallis; and U.S. Pat. No. 4,616,276 issued to Workman, teach various algorithmic methods of deriving feedforward signals in connection with disk drives.
The problem with all of the prior ail discussed above is that it does not provide an effective PES noise reduction method that has minimal impact on the required servo tracking stability. All noise filtering methods that use only past samples of the PES will adversely reduce the phase margin, and thereby reduce servo stability and cause poor tracking.